The following among the proteins so far tried, when used in suitable concentration, have induced normal growth in rats. Evidently all the essential amino-acids in suitable proportions are present in these proteins:

Proteins Of Animal Origin

Casein (found in milk); lactalbumin (milk); ovalbumin (hen's egg); ovovitellin (hen's egg). Proteins of vegetable origin. Edestin (hemp-seed); globulin (squash-seed); excelsin (Brazil-nut); glutelin (maize); globulin (cotton-seed); glutenin (wheat); glycinin (soy-bean); cannabib (hemp-seed).

Other proteins that have failed to induce growth are legumenin (soy-bean); vignin (vetch); gliadin (wheat or rye); legumin (pea); legumin (vetch); hordein (barley); conglutin (blue or yellow lupin); gelatin (horn); zein (maize); and phaseo-lin (white kidney bean).

All of the foodstuffs enumerated - milk, egg, wheat, etc. - always contain more than one of the proteins. So long as we eat mixtures of proteins we can know little of the biological value of each protein, except in rare instances, as in gelatin, which happens to be a homogeneous substance. Drs. Osborne and Mendel in the course of their various experiments have first very carefully isolated the various proteins from the foods.

Stunting

In connection with Drs. Osborne and Mendel's work, we may note one other experiment of theirs, because it touches upon the nature of growth. A number of rats were given a diet of such a nature as to prevent any appreciable gain or loss of weight (see chart 3, page 71, the first part of curve A). This "maintenance" diet was kept up for 550 days. At the end of this period the diet was so changed as to include all the necessary amino-acids; in other words, this second diet was a "growth diet." The rats immediately began to grow and gain in weight. The amazing feature of this experiment is that these rats began to grow 250 days after they would normally have reached maturity; for under normal conditions these same rats usually reach maturity at the end of 300 days. It is as if a boy had stopped growing when he was ten years old, and then by some freak would not continue his growth until he had reached his thirtieth year.*

Summary

To summarize the essential points that have been discussed in this chapter, we may say that 1. Maintenance (neither gain nor loss in weight) and growth both require a complex of amino-acids. Maintenance requires, among others, the amino-acid tryptophane and probably some lysine; growth, that of lysine, and, to a lesser extent, cystine.

2. Proteins cannot be used interchangeably with equal nutritive value. Their value to the body depends upon the kind and quantity of the individual amino-acids they contain. Because of this fact the problem of protein minimum resolves itself into a question of amino-acid minimum.

* I must, however, add that recent experiments by Dr. Jackson do not wholly confirm Drs. Osborne and Mendel's conclusions. While growth can be resumed after prolonged stunting, such growth is not as rapid as under normal conditions.

3. Stunting, whether by underfeeding in energy value of food supplied, by too low protein intake, or by absence of amino-acids necessary for growth, does not affect the inherent growth impulse.

It should be emphasized that these results have been reached by experiments on rats. How many such observations can be applied to man and his food cannot be stated with certainty; but if evidence were wanting that most of them are applicable, we need only point to the pitiable condition of many of the people in Europe. This condition is largely the result of poor nutrition or underfeeding. In either case amino-acids play an extremely important part. So do vitamines. But of that more anon.